prevent-input-zoom.user.js

// ==UserScript==
// @name         Prevent Input Zoom (robust)
// @match        *://*/*
// @run-at       document-start
// ==/UserScript==

const VIEWPORT_CONTENT =
  "width=device-width,initial-scale=1,maximum-scale=1,user-scalable=no";

function upsertViewportMeta() {
  let [meta, ...others] = document.querySelectorAll('meta[name="viewport"]');
  for (let otherMeta of others) {
    otherMeta.remove();
  }

  if (!meta) {
    meta = document.createElement("meta");
    meta.name = "viewport";
  }

  if (meta.getAttribute("content") != VIEWPORT_CONTENT) {
    // alert(meta.getAttribute("content"));
    meta.setAttribute("content", VIEWPORT_CONTENT);
  }

  if (!meta.parent) {
    document.head.appendChild(meta);
  }
}

function debouncedUpsertViewportMeta() {
  clearTimeout(debouncedUpsertViewportMeta.timeout);
  debouncedUpsertViewportMeta.timeout = setTimeout(upsertViewportMeta, 50);
}

debouncedUpsertViewportMeta();
new MutationObserver(debouncedUpsertViewportMeta) //
  .observe(document.head, {
    childList: true,
    subtree: false,
    attributes: true,
    attributeFilter: ["content", "name"],
  });